1. Field of the Invention
The invention relates to a normally-off type nitride semiconductor field effect transistor that is used in a high breakdown voltage power transistor.
2. Background Art
A GaN (gallium nitride) based compound semiconductor has a high breakdown field and a high saturation electron velocity and therefore is a very attractive semiconductor material for a future power transistor that requires a low on-state resistance and a high breakdown voltage. In an AlGaN/GaN (aluminum gallium nitride/gallium nitride) heterostructure formed on a substrate having a (0001) plane as a main surface, charges are formed at the heterointerface by spontaneous polarization and piezoelectric polarization, and a sheet carrier concentration of 1×1013 cm−2 or more is obtained even in an undoped state. A high current density HFET (Hetero-junction Field Effect Transistor) has been implemented by using this high concentration 2DEG (2-Dimensional Electron Gas) at the heterointerface. A conventional AlGaN/GaN HFET is a so-called normally-on type HFET in which a source-drain current flows when a drain voltage is applied with a gate electrode of 0 V.
From a standpoint of the safety in case of power outage, operation with a single circuit power supply, and the like, a power transistor to be used in practical applications should be a so-called normally-off type power transistor in which no current flows when a gate voltage is 0 V. A normally-on type power transistor has safety problems; for example, a circuit is damaged in case of power outage. Moreover, the normally-on type power transistor is operated by applying a positive voltage to a drain electrode and a negative voltage to a gate electrode. Therefore, two power supplies, that is, positive and negative power supplies, are required for the normally-on type power transistor. However, a normally-off type power transistor is operated by applying a positive voltage to both a drain electrode and a gate electrode. Therefore, the normally-off type power transistor can be operated with a single power supply.
It is desirable that, when a gate voltage is increased, a normally-off type FET has a sufficiently low gate leakage current until a drain current is saturated. In a conventional Schottky gate structure, however, a significant forward current flows at a gate voltage Vg of about +1 V, and therefore, a gate leakage current flows before a drain current is saturated. Accordingly, instead of the Schottky gate structure, a gate structure in which a gate leakage current is low even when a positive gate voltage is applied is required for the normally-off type FET.
Hereinafter, a conventional normally-off type GaN-based FET will be described. A method for forming a MIS (Metal Insulator Semiconductor)-type AlGaN/GaN HFET as a low gate leakage current element by forming an SiN (silicon nitride) insulating film under a gate electrode has been reported (e.g., see Japanese Laid-Open Patent Publication No. 2006-173294). Since SiN used as the insulating film has a wide bandgap, carriers are less likely to be transported between a gate metal and a semiconductor layer, and a gate leakage current can be reduced as compared to the Schottky gate structure.
It has been recently reported that, in such an AlGaN/GaN MIS-HFET, forming an insulating film of, e.g., SiN on an AlGaN barrier layer increases a sheet carrier concentration at the AlGaN/GaN heterointerface (e.g., see M. Higashiwaki, T. Matsui, “AlGaN/GaN Heterostructure Field-Effect Transistors with Current Gain Cut-off Frequency of 152 GHz on Sapphire Substrates,” J. J. Appl. Phys. vol. 44 (2005), L475; hereinafter, this document is referred to as Document 1). In Document 1, a high current MIS-HFET having excellent high frequency characteristics is implemented by making positive use of a high concentration 2DEG.
However, in order to implement both a low gate leakage current and a normally-off type operation, it is desirable that the sheet carrier concentration at the AlGaN/GaN interface does not increase in the MIS structure as well. In other words, in order to obtain a normally-off type FET while reducing a gate leakage current, it is necessary to compensate not only for polarized charges produced by a polarization effect but for increased sheet carriers in the MIS structure to deplete the AlGaN/GaN interface under a gate electrode.
A normally-off type AlGaN/GaN MIS-HFET in which a 2DEG at the AlGaN/GaN interface is depleted by implanting fluorine (F) ions into an AlGaN layer directly below a gate electrode by plasma processing and an SiN film is formed between the gate electrode and an AlGaN layer has been reported as a MIS-HFET that meets the above requirements (e.g., see R. Wang, Y. Cai, C. W. Tang, K. M. Lau, K. J. Chen, “Enhancement-Mode Si3N4/AlGaN/GaN MISHFETs,” IEEE Electron Device Lett., vol. 27, no. 10, pp. 793-795, October 2006).
However, detailed physical properties of the conventional normally-off type AlGaN/GaN MIS-HFET involving fluorine implantation have not been clarified. Moreover, the element is damaged by the plasma processing for fluorine implantation into the AlGaN layer, causing degradation in operation characteristics of the element or fluctuation for reliability through device processing.